temp = input('请输入成绩\n')
while not temp.isdigit():
    temp = input('请输入整数\n')
fen = int(temp)
if 60<fen<=80:
    print('C')
elif 80<fen<=90:
    print('B')
elif 0<fen<=60:
    print('D')
elif 90<fen<=100:
    print('A')
else :
    print('成绩不在范围内')
    temp = input('请重新输入\n')
